linux-kernel - Re: [PATCH] MIPS: implement smp_cond_load

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Message-ID: <20180619072242.GC2494@hirez.programming.kicks-ass.net>
Date:   Tue, 19 Jun 2018 09:22:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     陈华才 <chenhc@...ote.com>
Cc:     Paul Burton <paul.burton@...s.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <james.hogan@...s.com>,
        linux-mips <linux-mips@...ux-mips.org>,
        Fuxin Zhang <zhangfx@...ote.com>,
        wuzhangjin <wuzhangjin@...il.com>,
        Huacai Chen <chenhuacai@...il.com>,
        stable <stable@...r.kernel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        AndreaParri <andrea.parri@...rulasolutions.com>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: implement smp_cond_load_acquire() for Loongson-3

On Tue, Jun 19, 2018 at 02:40:14PM +0800, 陈华才 wrote:
> Hi, Paul,
> 
> First of all, could you please check why linux-mips reject e-mails
> from lemote.com? Of course I can send e-mails by gmail, but my gmail
> can't receive e-mails from linux-mips since March, 2018.

Could you please learn to use email? No top posting and wrap lines at 78
chars.

> I have already read Documentation/memory-barriers.txt, but I don't
> think we should define a smp_read_barrier_depends() for Loongson-3.
> Because Loongson-3's behavior isn't like Alpha, and in syntax, this is
> not a data-dependent issue.

Agreed, this is not a data-dependency issue.

> There is no document about Loongson-3's SFB. In my opinion, SFB looks
> like the L0 cache but sometimes it is out of cache-coherent machanism
> (L1 cache's cross-core coherency is maintained by hardware, but not
> always true for SFB). smp_mb() is needed for smp_cond_load_acquire(),
> but not every READ_ONCE().

Linux does _NOT_ support non-coherent SMP. If your system is not fully
coherent, you're out of luck.

But please, explain in excruciating detail what exactly you need that
smp_mb for. If, like I posited in my previous email, it is to ensure
remote store buffer flushes, then your machine is terminally broken.